Video signal modulator



Oct..-28, 1947. A c. E. H'ALLMARK f I' 2,429,755 v VIDEO SIGNAL MODULA'TR v Filed ocfc.'14, 1944 INVENTOR Patented Oct. 28, 1947 vIDEo SIGNAL MoDULA'roR Clyde-E. Hallmark, Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation, a corporation of Delaware Application October 14, 1944, Serial No. 558688 3 Claims.

This invention relates to radio transmitting apparatus and particularly to modulating systems employed in such apparatus.

It is the conventional practice in radio transmitters to modulate the output energy of a power amplier by impressing a modulating signal upon the amplifier. InV one type of system the modulating signal is impressed upon the control grid circuit of the power amplifier to eiect what is known as grid modulation. For this purpose the output circuit of a modulating tube is coupled to the power amplifier. The usual arrangement is to couple the anode circuit of a modulating tube to the grid circuit of the power amplier. In view of the fact that such amplifiers generally are operated class C, it is necessary to biasV the grid circuit negatively with respect to ground. Inasmuch as the anode circuit of the modulating tube usually is, at. all times, at a positive potential with respect to ground, it is necessary to include in the coupling circuit an isolating or blocking condenser so that the positive direct current present in the anode circuit kof the modulating tube will not affect the negative direct current bias applied to the power amplifier grid circuit. Y

In some cases, such as in' a television system, however, itis desirable to1modulate-the output of a power' ampliernnt: only with the alternating currentY componentof the intelligence signal but also with the direct; current component thereof. In thaconventional arrangementutilizing a coupling condenser between thezanode circuit of the modulating tubeand :thegridcircuitof the. power amplifier, it is possibletoffmcdulater the power amplifier only in accordaizice v-witlfi` .thealternating component of. the intelligence signalfor. the reason that the direct current' component will not be transferred through the couplingcondenser. Furthermore', particularly in. a television` system, the video signal is preeamplied before it isused to control the modulating tube. Inasmuch as it is preferred generallyto use coupling circuits of the alternating current type: between stages of the pre-amplifier, the direct current component of the Video signal, if present in the signal as generated, is lost in the amplifier.

It is an object of the present invention, therefore, to provide apparatus-for modulating a power amplier in accordance with both the alternating current and the direct current components of an intelligence signal.` y

According to this invention, there is provided a modulating tube having` a cathode follower output circuit which is coupled to a power amplifier so as to control the energy output of the amplifier in accordance with an intelligence signal developecl in the output circuit of the modulating tube. In the case where the intelligence signal is to be pre-amplified before it is impressed upon the modulating tube, it is first modulated on a sub-carrier. The modulated sub-carrier then may be amplied as desired by means of conventional ampliers employing alternating current types of couplings between stages. The amplied sub-carrier then is impressed upon a rectifier by means of which there are recovered both the alternating current and the direct current'componente of the intelligence signal. By means of such a system the power amplifier then is capable of being modulated by both alternating current and direct current components of the signal.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

The single figure of the accompanying drawing is a schematic illustration of apparatus embodying one form of the invention.

Having reference now to the drawing, the modulating system comprises a source I of signalmodulated sub-carrier wave energy. Such apparatus may be entirely conventional, consisting of an oscillator, the generated energy of which is modulated in amplitude by the intelligence signal. In the output circuit of such an oscillator there may be included a circuit comprising an inductor or coil 2 tuned for resonance at the frequency of the sub-carrier by means of a shunting condenser 3. Inductively coupled to the coil 2 is another coil or inductor 4 which is shunted by a load resistor 5. The terminals of this resistor are connected to the input or control circuit of a grid controlled rectiiier tube 6. The cathode of this tube is connected through an output or load. resistor 'l to the negative terminal of a power source such as a battery 8, the positive terminal of which is connected to ground. The anode of thev rectifier tube also is connected to ground. The load resistor 'l is shunted by a condenser 9 to by-pass the carrier frequency around the resistor.

The terminals of the rectifier load resistor l are conductively coupled to the input circuit of a modulator tube I 0. Specifically, the cathode terminal of the resistor 'I is connected through an inductor Il to the control grid of the modulator tube. The cathode of the modulator tube is connected through an output or load resistor I2 to an intermediate point on the battery 8 so as to properly bias the control grid of this tube. The anode of the modulator tube is connected to ground.

The apparatus also includes a powerA amplifier of a conventional type which, as illustrated, consists of two tubes I3 and I4. The cathodes of these tubes are connected to the terminals of an inductor or coil I5 which is shunted by a tuning condenser I6. Inductively coupled to the coil l5 is a coil I'I, the terminals of which are connected to a source of radio frequency carrier I8. The resonant circuit comprising the coil I5 and the condenser IG is tuned to the radio frequency carrier so that the power amplifier tubes I3 and i4 may be excited at this frequency. The anodes of the power amplifier tubes are connected to the respective terminals of the output coil I9 with which there is connected in parallel a timing condenser 2D. The resonant circuit thus formed is also tuned to the frequency of the radio frequency carrier. Power is supplied to the amplifier from any convenient source such as a battery 2|, the positive terminal of which is connected to the midpoint of the output circuit coil I9 and the negative terminal of which is connected to ground and the midpoint of the exciting circuit coil I5. In this manner it is seen that there is established a direct current coupling between the anode of the modulating tube li! and the cathode circuit of the power amplifier including the tubes I3 and I4. A utilization circuit 22 is connected to a coil 23 which is inductively coupled to the output circuit coil I9 of the amplifier.

The cathode terminal of the modulator tube load resistor I2 is connected through an inductor or coil 24 to the control grids of the power amplifier tubes I3 and I4, thereby establishing a direct current coupling between the modulator tube cathode and the power amplifier grid circuit. These grids are by-passed to ground by a condenser 25.

Referring now to the operation of the described apparatus, the sub-carrier energy which is modulated in amplitude in accordance with a video signal of a television system, for example, is impressed upon the input circuit of the rectifier.

tube 6. As a consequence, the demodulated signal is developed in the load resistor 'I comprising the cathode follower output circuit of the tube f5. The sub-carrier frequency is by-passed around this resistor through the condenser 9. Further filtering of the sub-carrier from the output circuit of the rectifier is accomplished by the coil II by means of which the demodulated signal impressed upon the input circuit of the modulating tube.

modulating tube a signal voltage suitable to control the energy output of the power amplifier. The conductivity of the amplifier tubes lC and Thus, there is developed in the cathode follower output circuit resistor I2 of the I4 is varied in accordance with the modulatingA signal so that the energy developed in the output quency wave. It follows then that the signal which is developed in the rectifier output circuit resistor 1 will include both the alternating current and the direct current components of the signal, Likewise, 'by means of the conductive coupling between the rectifier tube and the modulating tube and also between the modulating tube and the control grid circuit of the power amplifier, the direct current component of the signal may be impressed upon the power amplifier.

It is seen that, by reason of the cathode connections of the power amplier tubes I3 and I!! to the terminals of the coil I5 which has the mid-point thereof grounded, these cathodes are maintained substantially at ground potential for direct current. Likewise, by reason of the connection of the power amplifier grids through the coil 24 and the resistor I2 to a point on the battery 8 which is negative with respect to ground, the power amplifier grids may be operated sufciently negative with respect to their associated cathodes to provide the proper biasing to effect class C operation. At the same time the power amplier grids are subject to the control of the video signals including both the direct current and the alternating current components by reason of their conductive connection to the output circuit resistor I2 of the modulator tube. It thus is seen that, by reason of deriving the signal voltages from the cathode follower output circuit of the modulator tube, it is possible to provide a conductive coupling between the output circuit of the modulator tube and the input circuit of the power amplifier and at the same time to provide the necessary biasing of the power amplifier input circuit to effect the desired class C operation.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a modulating system, a source of signal modulated sub-carrier frequency energy, a rectifier tube coupled to said source of sub-carrier frequency energy and having a cathode follower circuit, a modulating tube coupled to the cathode follower circuit of said rectifier, said modulating tube also having a cathode follower circuit, and a power amplifier having a grid controlled input circuit conductively coupled to the cathode follower circuit of said modulating tube.

2. In a modulating system, a source of signal modulated sub-carrier frequency energy, a rectifier tube coupled to said source of sub-carrier frequency energy and having a cathode follower circuit, a modulating tube having an input circuit conductively coupled to the cathode follower circuit of said rectifier, said modulating tube also having a cathode follower circuit, a power amplifier having a grid controlled input circuit conductivelycoupled to the cathode follower circuit of said modulating tube, and a source of direct current energy connected in the cathode follower circuit of said modulating tube.

3. In a modulating system, a, source of signalmodulated sub-carrier frequency energy, a rectifier tube having a grid controlled input circuit coupled to said source of sub-carrier frequency energy and a cathode follower circuit, a modu- 5 6 lating tube having a, grid controlled input circuit REFERENCES CITED conductively coupled to the cathode follower circuit of said recter, said modulating tube also llff gvgggrens are of record in the having a cathode follower circuit, a, power amf plier having a grid controlled signal input cir- 5 UNITED' STATES PATENTS cuit conductively coupled to the cathode follower Number Name Date circuit of said modulating tube, and a source of 2,049,424 Coojy Aug. 4 1936 direct current energy connected to supply energy 2,163,167 Weafrant June 20, 1939 to the cathode follower circuit of said modulating tube and a negative biasing voltage to the input 10 FOREIGN PATENTS ycircuit of said power amplifier. Number Country Date 424,470 Great Britain Feb. 21, 1935 CLYDE E- HALLMARK 87,667 sweden oet. 2o, 193s 

